1. Field of the Invention
The present invention relates to novel precipitated silica particulates, and, more especially, to certain hydrophilic precipitated silica particulates well adapted for use as a reinforcing filler for the curable organopolysiloxanes and for other elastomers.
2. Description of the Prior Art
It has long been known to this art to prepare precipitated silica particulates by acidifying an aqueous alkali metal silicate solution, but it was quickly recognized that, in spite of the apparent simplicity of such process, the silica particulates thus prepared were not imbued with satisfactorily attractive overall properties as to enable their use as a desirable reinforcing filler for the various elastomers.
And in spite of repeated efforts to ascertain and coordinate the proper combination of such parameters as determining factor, BET specific surface area, roughness or coalesence factor, oil absorption, and structural index, the resulting filler properties remained far inferior to those imparted by pyrogenic silicas which unfortunately are much too expensive to be of practical use.
Thus, it has long been attempted, in order to enhance the adaptability of the silica particulates for the purpose of reinforcing elastomers, to develop various processes for preparing precipitated silicas, which attempts became increasingly complex and sophisticated, and wherein the temperature, levels of concentration of the reactants (silicate, acid, etc.) the flow rates of the reactants, the sequences of successive reactant addition operations, the pH, etc., had to be very carefully controlled. Compare, for example, French Pat. No. 1,352,354; U.S. Pat. Nos. 3,954,944 and 4,127,641.
Attempts were also made, moreover, to improve the reinforcing nature of precipitated silicas for silicone applications, by rendering the silica particulates hydrophobic by suitable surface treatment (using, for example, a silane, silazane, or like agent). Hydrophilic silicas which have been rendered hydrophobic by means of such a treatment and which can be used as reinforcing fillers for the silicones are described, for example, in French Pat. No. 2,356,596.
Finally, and quite recently, there has also been developed a precipitated silica which is greatly improved filler-wise, but which imparts to organopolysiloxane elastomers, properties of reinforcement which are considerably poorer than those imparted by the pyrogenic silicas (International Rubber Conference held at Kiev, Oct. 10-14, 1978).
Cf. French Pat. Nos. 1,453,198, 1,483,791, 1,150,836, 2,218,285 and 2,353,486; British Pat. No. 883,863.